Re: Interesting question on const.

From: Dag-Erling Smørgrav (des_at_des.no)
Date: 03/24/05 

Date: Thu, 24 Mar 2005 00:43:04 +0100

"Tor Rustad" <torust@online.no> writes:
> <vashwath@rediffmail.com> wrote in message
> > Recently I attended an interview in which the question
> > "Is there any difference between "const T var" and
> > "T const var"?
> The answer is that it depends.

No.

> Let say T is a typedef for "int", then we have
>
> const int var; /* declare read-only int */
> int const var; /* declare read-only int */
>
> which is exactly the same thing. "const" type-qualify
> it's left, but the special case of "const" being first in
> the declaration, it type-qualify it's right.

Yes.

> Now let say T is a typedef for "int *". Then we have
> these two cases:
>
> int * const var; /* declare read-only int pointer */
> const int *var; /* declare pointer to read-only int */
>
> In the first case, the int pointer is const, while in the
> letter case, int is const. This isn't the same thing!

No. The const qualifier applies to the entire typedef. Both variants
are interpreted as "int * const var".

The best way to avoid this confusion is to always place const on the
right of the type it qualifies.

If T were an unparanthesized macro, and not a typedef, your answer
would be correct, but I'm pretty sure that's not what the OP was
asked, and in all likelihood, the interviewer was following a script
and would not have understood a complete answer.

Here's a program which demonstrates this:

 1: typedef int * T;
 2: const T a;
 3: T const b;
 4: int main(int argc, char *argv[])
 5: {
 6: a = &argc;
 7: *a = 1;
 8: b = &argc;
 9: *b = 1;
10: return 0;
11: }

As written, it results in the following errors:

const.c: In function `main':
const.c:6: error: assignment of read-only variable `a'
const.c:8: error: assignment of read-only variable `b'

if you replace the first line with

 1: #define T int *

you get the following:

const.c: In function `main':
const.c:7: error: assignment of read-only location
const.c:8: error: assignment of read-only variable `b'

DES 

-- 
Dag-Erling Smørgrav - des@des.no

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